1. Field of the Invention
The present invention generally relates to a flow sensor for measuring the flow rate of gaseous media, such as air, and relates more particularly to a multiple technology flow sensor suitable for measuring various physical characteristics of gaseous media, such as pressure, temperature, and flow rate, which may be used to enhance the accuracy of flow rate measurements.
2. Description of the Prior Art
There are essentially three prevailing methods used to measure the flow of gaseous media. The first is inferential flow measurement, which senses a difference in pressure across a restricted orifice. The second method uses a thermal sensor, which is also referred to as a constant temperature anemometer, to monitor temperature changes that are dependant upon the speed of the medium. This method is described in U.S. Pat. No. 6,470,741 to Fathollahzadeh, which is incorporated herein by reference. The third method utilizes displacement sensors that detect mechanical displacement of a portion of the sensor caused by the flow of gas.
Each of these methods has different application ranges, as well as inherent advantages and disadvantages. Inferential flow measurement generally requires two pressure sensors and a restriction in flow. A temperature sensor is also typically required with this method to compensate for variations in pressure due solely to temperature fluctuations. However, the requirement of multiple sensors substantially increases the rate of failure and cost of installation. Thus, application of inferential flow measurement principles becomes practical in only limited circumstances.
The remaining two flow sensing methods do not require multiple sensors, but have other drawbacks. Typically, thermal sensors are used for lower flow rates while displacement sensors are used for relatively higher flow rates. Selection of the most appropriate flow sensor for a particular application requires a detailed knowledge of the anticipated range of measurements, the potential physical characteristics of the medium, such as temperature and pressure, as well as the environmental characteristics of the location in which the medium is to be measured. Accordingly, use of either thermal sensors or displacement sensors generally requires a customized solution for each particular application.
Conventional methods of measuring flow rate typically involve the use of separate dedicated sensors. These sensors are often located at significant distances from each other, which necessitate the use of external wiring and/or interface assemblies, as well as substantially increasing the size, cost, and space requirements of the system.
In addition, most flow rate sensors do not compensate for the effect of temperature or pressure in the flow rate measurement. Accordingly, such measurements may be highly inaccurate, particularly when taken over a wide range of conditions.